Add a singleton id generator util (#2815)

* Add a singleton id generator utility

core/utils/idgenerator.js

@@ -0,0 +1,67 @@
+/**
+ * @license
+ * Visual Blocks Editor
+ *
+ * Copyright 2019 Google Inc.
+ * https://developers.google.com/blockly/
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * @fileoverview Generator for unique element IDs.
+ * For UUIDs use Blockly.utils.genUid. The ID generator should primarily be
+ * used for IDs that end up in the DOM.
+ * @author samelh@google.com (Sam El-Husseini)
+ */
+'use strict';
+
+
+goog.provide('Blockly.utils.IdGenerator');
+
+
+/**
+ * Creates a new id generator.
+ * @constructor
+ * @final
+ */
+Blockly.utils.IdGenerator = function() {};
+
+/**
+ * Get the singleton instance of Blockly.utils.IdGenerator.
+ * @returns {Blockly.utils.IdGenerator} singleton instance
+ */
+Blockly.utils.IdGenerator.getInstance = function() {
+  if (!Blockly.utils.IdGenerator.instance_) {
+    Blockly.utils.IdGenerator.instance_ = new Blockly.utils.IdGenerator();
+  }
+  return Blockly.utils.IdGenerator.instance_;
+};
+
+/**
+ * Next unique ID to use
+ * @type {number}
+ * @private
+ */
+Blockly.utils.IdGenerator.prototype.nextId_ = 0;
+
+/**
+ * Gets the next unique ID.
+ * The difference between this and genUid is that getNextUniqueId generates
+ * IDs compatible with the HTML4 id attribute restrictions:
+ * Use only ASCII letters, digits, '_', '-' and '.'
+ * @return {string} The next unique identifier.
+ */
+Blockly.utils.IdGenerator.prototype.getNextUniqueId = function() {
+  return 'blockly:' + (this.nextId_++).toString(36);
+};